If an instance is defederated, the owners can just spin up a new instance.
I've always thought about what you've said about Lemmy when people start talking about how Lemmy is more privacy focused than Reddit.
As one of your replies have said many people in the hundreds/thousandths have a copy of your data on Lemmy - the instance owners. If you decide you've shared too much information then you end up asking every owner to delete that nugget of information. And realistically there is nothing to enforce it. This is one benefit of the walled garden of places like Reddit because they are legally obligated to delete the information especially in places like the EU.